1. Field of the Invention
This invention refers to telecommunication systems and it concerns in particular a satellite telecommunication system.
2. Description of the Related Art
Within the sphere of telecommunications, stationary satellites cover an ever more important role, both for television connections and for all the other applications, where they are used as transponders, for example in telephone communications.
As it is known, these satellites are located at a height of approximately 36,000 km in an orbit contained in the plane passing by the terrestrial equator. At this height, the speed, which this satellite must have to keep its position in the orbit, nullifying the gravitational pull force coincides with the earth rotation speed. The final result for an observer on the Earth, is that of seeing the fixed satellite in a precise position on the horizon. The choice of the orbital position (15 ° East, 28° West etc.) is made to stay, as much as possible, over the regions on the earth surface where the television signal is to be sent. By way of example, the position of one of the Hot Bird™(13° East) satellites, used to broadcast television signals, is optimal to cover Central Europe. Similarly, orbital positions shifted westwards are used for the Americas, and orbital positions shifted eastwards for Australia.
To ensure the best quality of signals received by earth stations, satellites are equipped with antennas having a radiation pattern shaped to concentrate transmitted power towards areas to be served.
Satellite position on the celestial vault, as seen by the land station is defined by the azimuth and the elevation angles, referred to the geographic North and to the horizon plane respectively, and it obviously varies according to the geographical co-ordinates of the land station itself. In particular, moving towards the poles, the angle between the horizon and the satellite direction, is gradually reduced, while the latitude increases (both Northwards and Southwards).
For this reason, to receive signals from a stationary satellite in the regions near the Equator, it is necessary to aim the antenna almost to the zenith (should the orbital position not coincide with the zenith of the land station, this angle must be recovered by slightly tilting the antenna).
Referring to a nation like Italy and to a stationary satellite like Hot Bird (13° East), the elevation required at Bolzano is of 36.3° and at Ragusa it is of 47.1° (example of maximum and minimum latitude for Italy), with a total excursion of approximately 10 degrees. The use of antennae with a sufficiently symmetrical radiation diagram with respect to the axis, entails therefore the necessity of having to set, at the time of the installation, both the elevation and the azimuth. This is currently performed at the installation of antennae, such as parabolas, both at a position that is fixed or a position that can be remote-controlled by means of mechanical positioning, see by way of example Patent EP0838876 application, or electronic controlled antennae at array with scanning beam, see by way of example U.S. Pat. No. 6,184,827.
Were it is possible to manage just the azimuth angle, installation operation would be simpler, and hence cheaper, and it would be possible to aim at different satellites without changing the elevation.